1. Field of the Invention
The present invention relates to an electromechanical transducer.
2. Description of the Related Art
In recent years, electromechanical transducers produced by a micromachining process have been researched actively. In particular, capacitive electromechanical transducers called capacitive micromachined ultrasonic transducers (CMUT) have attracted attention, because they can transmit and receive ultrasonic waves with a lightweight membrane and can obtain wider band characteristics than piezoelectric electromechanical transducers of the related art.
A CMUT includes a plurality of elements arranged in an array in a one-dimensional or two-dimensional direction. Elements serve to transmit and receive ultrasonic waves. FIG. 11A is a schematic top view of a CMUT of the related art. An element 301 shown in FIG. 11A includes a plurality of cells 311. By simultaneously applying a driving voltage signal to the cells 311 of the element 301, ultrasonic waves are output from the element 301. Further, ultrasonic detection signals received by the cells 311 of the element 301 are added by upper electrodes 315 and a lower electrode (not shown) that is common to the cells 311, and the sum serves as an ultrasonic detection signal received by the element 301. The upper electrodes 315 in the cells 311 are electrically connected by lines 307.
U.S. Pat. No. 6,958,255 discloses an example of a CMUT having such an element structure. In this CMUT, a substrate penetrating line 304 is provided in a support substrate 303, as shown in FIG. 11B. A circuit board 305 is electrically connected to a lower electrode 316 by the substrate penetrating line 304, and is electrically connected to upper electrodes 315 by lines, an insulating-layer penetrating line, and the substrate penetrating line 304. In the circuit board 305, driving voltage signals are generated to output an ultrasonic wave from an element, and an ultrasonic signal generated by an ultrasonic wave received by the element is subjected to processing such as amplification and delay addition.
Unfortunately, the displacement amount of the membrane varies among the cells of the element. It can be conceived that this variation among the cells is caused by warping of the substrate due to the difference in coefficient of thermal expansion between the membrane and the insulating layer and internal stresses of the membrane and the insulating layer. The variation is undesirable because it appears as differences in transmission efficiency and detection sensitivity for the ultrasonic wave.
The transmission efficiency and detection sensitivity of the CMUT increase as the gap between the upper and lower electrodes decreases. Since electrostatic attractive force between the upper and lower electrodes is increased by increasing the bias voltage, the transmission efficiency and detection sensitivity of the CMUT can be enhanced by increasing the bias voltage. However, when the bias voltage excessively increases, the upper electrode is attracted to the lower electrode together with the membrane the instant that the bias voltage reaches a certain voltage, so that it is difficult to obtain a desired vibration characteristic. This phenomenon is referred to as a pull-in, and a voltage at which a pull-in occurs is referred to as a pull-in voltage. A pull-in voltage is determined by the initial displacement amount of the membrane. Thus, since the upper limit value of the bias voltage applied between the upper and lower electrodes is limited by variation in initial displacement amount of the membrane among the cells, the receiving sensitivity of the CMUT is limited.